Age-related macular degeneration (AMD) is the leading cause of permanent vision loss among the elderly in many industrialized countries. Smith, W., Assink, J., Klein, R., Mitchell, P., Klayer, C. C., Klein, B. E., Hofman, A., Jensen, S., Wang, J. J. & de Jong, P. T. (2001) Ophthalmology 108, 697-704. The majority of vision loss due to AMD is a result of pathologic new blood vessels, termed CNV, invading the retina from the underlying choroid through fractures in Bruch membrane, the extracellular matrix between the choroid and the retinal pigment epithelium (RPE). The earliest clinical hallmark of AMD is the appearance of drusen (Gass, J. D. (1972) Trans Am Ophthalmol Soc 70, 409-36), localized lipoproteinaceous deposits between the RPE and Bruch membrane. Although their presence is an epidemiological risk factor for the development of CNV (Bressler, S. B., Maguire, M. G., Bressler, N. M. & Fine, S. L. (1990) Arch Ophthalmol 108, 1442-7; and Macular Photocoagulation Study Group (1997) Arch Ophthalmol 115, 741-7), the mechanism of how, or if, drusen provoke CNV remains undefined. Some investigators have suggested that drusen are epiphenomena, while others have claimed that drusen constituents act as a focal stimulus for inflammatory cells that secrete angiogenic molecules such as VEGF, and still others have suggested that drusen disturb RPE homeostasis by impairing transport across Bruch membrane. Reviewed in Ambati, J., Ambati, B. K., Yoo, S. H., Ianchulev, S. & Adamis, A. P. (2003) Surv Ophthalmol 48, 257-293.
Recent work has demonstrated that complement components C3 and C5 are principal constituents of drusen in patients with AMD. Mullins, R. F., Russell, S. R., Anderson, D. H. & Hageman, G. S. (2000) FASEB J 14, 835-46; Johnson, L. V., Ozaki, S., Staples, M. K., Erickson, P. A. & Anderson, D. H. (2000) Exp Eye Res 70, 441-9; Anderson, D. H., Mullins, R. F., Hageman, G. S. & Johnson, L. V. (2002) Am J Ophthalmol 134, 411-31; and Leitner, W. P., Staples, M. K. & Anderson, D. H. (2001) Exp Eye Res 73, 887-96. Their presence as well as that of the membrane-attack-complex (MAC) C5b-9 and other acute phase reactant proteins in RPE cells overlying drusen has fueled speculation that drusen biogenesis involves chronic inflammatory processes that either can trigger complement activation and formation of MAC acting to lyse RPE cells or disturb physiological homeostasis in RPE cells. Johnson, L. V., et al. (2001) Exp Eye Res 73, 887-96.
Recently we described an animal model of AMD in aged Ccl2−/− and Ccr2−/− mice, which develop many salient pathological features seen in the human condition. Ambati, J., Anand, A., Fernandez, S., Sakurai, E., Lynn, B. C., Kuziel, W. A., Rollins, B. J. & Ambati, B. K. (2003) Nat Med 9, 1390-1397. Interestingly, RPE and choroidal deposits of C3 and C5 also are found in these mice at a young age. The inability of these mice, which are impaired in induced macrophage trafficking, to clear these complement deposits is thought to promote the later development of CNV via upregulation of RPE cell secretion of VEGF. Id. These findings suggest a mechanistic link between deposition of complement components in drusen and the development of CNV.
There is growing evidence that complement components are more than mere mediators of innate immunity. To date, their influence on the molecular regulation of angiogenesis in vivo has not been addressed. Because drusen predispose to and predate CNV, we explored whether C3a and C5a (the activated forms) play a role in CNV development by studying their impact on VEGF expression, and also in laser-induced CNV, an accelerated model of neovascular AMD that reproduces much of the pathology and immunophenotype of human CNV (Reviewed in Ambati, J., et al. (2003) Surv Ophthalmol 48, 257-293), using mice deficient in receptors for C3a or C5a. The effect of modulating C3a and C5a reveals novel therapeutic strategies to modulate angiogenesis in the setting of inflammation and highlights the importance of developing the ability to assay expression of markers such as C3a and C5a to for diagnostics and to target therapeutics more specifically.